1. Field of the Invention
The present invention relates to a speaker unit, and more particularly to a speaker unit compatible with an audio/visual device (hereinafter called an A/V device).
2. Description of the Prior Art
A conventional speaker unit compatible with an A/V device is provided with a magnetic shield cover for suppressing adverse magnetic influence of the speaker unit upon a television screen. Such speaker unit is, for example, disclosed in U.S. Pat. No. 4,465,908. An example of such a speaker unit is shown in FIG. 1. A magnetic circuit unit 1 is covered with a magnetic shield cover 2 made of iron sheet and formed generally in a tubular shape with a bottom, thereby shielding magnetic flux from the magnetic circuit unit 1. The magnetic shield cover 2 is mounted on a second magnet 5 upon which a yoke 3 and a first magnet 4 opposite in polarity to the second magnet 5 are mounted. In the figure, reference 6 represents a top plate, references 8, 9, 10 and 11 respectively represent a voice coil bobbin, diaphragm, damper and dust cap.
In the speaker unit described above, the second magnet 5 is disposed only for the purpose of magnetic shielding and does not serve as an element of the magnetic drive circuit, which makes the unit uneconomical.
Furthermore, a speaker in general has an impedance rise at about f.sub.0 in its enclosure characteristic. It is known in the art that such characteristic causes a considerable deterioration in fidelity at a low frequency.
In order to improve this, a speaker system has been proposed which used two stack-wound voice coils, one coil being supplied with a usual signal current and the other coil with a control signal current. With this speaker system, however, there is a limit to the quantity of magnetic energy at the gap where the two voice coils are disposed. Also, a large gap is required because of an increase in coil thickness. Thus, it can be understood easily that a remarkable effect cannot be expected for such system.